doc BUGFIX trees information organization

Remove duplicated groups of schema information and improve overall
organization and coverage for schema and data groups in doxygen.
diff --git a/src/tree_data.h b/src/tree_data.h
index b3a2ab4..5d7d480 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -31,6 +31,7 @@
 
 /**
  * @defgroup datatree Data Tree
+ * @ingroup trees
  * @{
  *
  * Data structures and functions to manipulate and access instance data tree.